The present invention relates to improvements in methods of manufacturing dielectric ceramic compositions for electronic devices, and in particular, to a method of manufacturing dielectric ceramic compositions for electronic devices comprising composite perovskite type compounds utilized in the SHF band whereby a ceramic is obtained of internally uniform properties, in particular of excellent temperature coefficient and sintering ability, by controlling the coupling conditions of metallic ions and oxygen ions in the composition by using a specific content of the oxygen concentration in the sintering atmosphere.
Dielectric ceramic compositions for electronic devices of various types are utilized on account of their low loss characteristics and excellent temperature characteristics. There are various types of such dielectric ceramic compositions, which are used in particular for temperature compensating capacitors and in dielectric resonators or strip line cards for microwave device etc for satellite broadcasting and reception and for down-conversion, utilizing their low loss in the SHF band.
In general, as dielectric ceramic compositions employed for the SHF band, conventional perovskite compounds which were widely employed included composite perovskite compounds in particular Ba(B⅓.A⅔)O3 compositions (where A is Ta and B is a divalent metallic ion (Zn or Ni, Co, Mn)), specifically Ba(Zn⅓.Ta⅔)O3 materials.
The characteristics such as high xcex5r, high Q and xcfx84f=0 that are required for the dielectric ceramic compositions used in this SHF band are tightly specified and composition control in order to match such characteristics that are needed by materials capable of f control is extremely important; for this purpose, prolonged sintering and complex adjustment of constiutents was necessary.
In conventional dielectric ceramic compositions, composition control, in particular, since the Zn contained in the aforesaid composition easily evaporates, control of Zn, is extremely important. Also, on sintering, the Zn diffuses to the outer surface of the ceramics and is volatilized, tending to form a so-called xe2x80x9cskinxe2x80x9d due to the formation of Zn-deficient constituents, such as Ba5Ta4O15. Thus, it was difficult to obtain internally uniform ceramics in stable fashion, so it was difficult to obtain ceramics of stable characteristics.
Also, depending on the application, it was necessary to adjust to a prescribed resonance frequency temperature coefficient xcfx84f; however, it was known that the Ba(Zn⅓.Ta⅔)O3 material has a if in the vicinity of zero.
Previously, as a result of various investigations of the aforesaid Ba(Zn⅓.Ta⅔)O3 etc composition aimed at eliminating the drawbacks of the aforesaid composite perovskite compounds and controlling the Zn in the dielectric ceramic composition, by controlling the Zn to a prescribed value in the composition by the inclusion of specific trivalent metallic ions, as ceramics of internal uniformity with excellent characteristics and excellent sintering capability,
XBa(Zn⅓.Ta⅔)O3xe2x80x94YSr(Gaxc2xd.Taxc2xd)O3 solid solutions and XBa(Zn⅓.Ta⅔)O3xe2x80x94Y(BaZ.Sr1-Z)(Gaxc2xd.Taxc2xd)O3 solid solutions have been proposed (Laid-open Japanese Patent Application No. H. 2-285616, Laid-open Japanese Patent Publication No. H. 3-232751).
In recent years, with miniaturization of electronic devices and the shift of frequency bands in the communications field to high frequency bands, the drawback has been experienced that it has been necessary to make dielectric elements of large size, owing to the low dielectric constant of conventional dielectric ceramic compositions.
In view of the foregoing situation regarding conventional composite perovskite compounds, an object of the present invention is to provide a method of manufacturing a dielectric ceramic composition for electronic devices whereby an internally uniform ceramic composition of excellent relative permittivity can be obtained in a stable fashion by controlling the temperature coefficient of the resonance frequency, by adjusting the oxygen concentration of the sintering atmosphere to a specific content, without adjusting the composition, by suppressing evaporation of Zn contained in the ceramic composition.
Aiming at solving the above problems of the prior art, the present inventors discovered that the dielectric constant of a system in which some of the Ta in the foregoing Ba(Zn⅓.Ta⅔)O3 solid solution system was substituted by Nb, i.e. an XBa(Zn⅓.(TaM.Nb1-M)⅔)O3xe2x80x94Y(BaZ.Sr1-Z)(Gaxc2xd.Taxc2xd)O3 solid solution system was increased, and the Ga in a YSr(Gaxc2xd.Taxc2xd)O3 solid solution system was effective in improving sintering facility.
However, although the molding was cooled with a cooling rate of 250xc2x0 C./Hr to room temperature after holding for 30 minutes to 96 hours at 1400xc2x0 C. to 1550xc2x0 C. after heating to 1400xc2x0 C. to 1550xc2x0 C. with a rate of temperature rise of 250xc2x0 C./Hr in an atmosphere of O2 concentration of 70% or higher, the drawback was experienced of a considerable fall in relative permittivity when the temperature coefficient of resonance frequency was controlled to a low value by adjustment of the composition of this solid solution ceramic composition.
However, as a result of various investigations aimed at preventing this drop in relative permittivity of the solid solution system, the inventors discovered that the temperature coefficient of resonance frequency could be controlled and the drop in relative permittivity restricted to a minimum by, instead of adjusting the composition of the solid solution system, adjusting the oxygen concentration in the sintering atmosphere to a specific content, and in this way perfected the present invention.
Specifically, when manufacturing a dielectric ceramic composition for electronic devices consisting of a composition expressed by the basic composition X(BaZ.Sr1-Z)(Zn⅓(TaMNb1-M)⅔)O3xe2x80x94Y(BaZxe2x80x2.Sr1-Zxe2x80x2)(Gaxc2xd.Taxc2xd)O3 wherein X, Y, Z, Zxe2x80x2 and M that define the composition ranges satisfy the following values, the present invention consists in a method of manufacturing a dielectric ceramic composition for electronic devices wherein sintering is performed at 1400xc2x0 C. to 1550xc2x0 C. or sintering is performed at 1400xc2x0 C. to 1550xc2x0 C. after pre-sintering at 900xc2x0 C. to 1300xc2x0 C. in an N2 atmosphere containing an oxygen concentration of 6% to 40%:
X+Y=1,
0.3xe2x89xa6Xxe2x89xa61,
0.7xe2x89xa7Yxe2x89xa70,
0xe2x89xa6Zxe2x89xa61,
xe2x80x830xe2x89xa6Zxe2x80x2xe2x89xa61,
0.2xe2x89xa6Mxe2x89xa60.8.
According to the present invention, BaCO3, SrCO3, ZnO, Ta2O5, Nb2O5, Ga2O3 powder of grain size of 1 xcexcm or less are blended and mixed so as to form a composition X(BaZ.Sr1-Z)(Zn⅓(TaM.Nb1-M)⅔)O3xe2x80x94Y(BaZxe2x80x2.Sr1-Zxe2x80x2)(Gaxc2xd.Taxc2xd)O3(X+Y=1, 0.3xe2x89xa6Xxe2x89xa61, 0.7xe2x89xa7Yxe2x89xa70, 0xe2x89xa6Zxe2x89xa61, 0xe2x89xa6Zxe2x80x2xe2x89xa61, 02xe2x89xa6Mxe2x89xa60.8), then dried and calcined in air or an oxygen or N2 atmosphere at 1000xc2x0 C. to 1200xc2x0 C., then pulverized and granulated, uniaxially pressure-molded, or molded under hydrostatic pressure, then subjected to removal of binder, and heated with a rate of rise of temperature of 50xc2x0 C./hour to 300xc2x0 C./hour in an N2 atmosphere of oxygen concentration adjusted to 6% to 40%, and sintered by holding for 30 minutes to 96 hours at 1400xc2x0 C. to 1550xc2x0 C., then cooled with a cooling rate of 50xc2x0 C./hour to 300xc2x0 C./hour to room temperature.
In the present invention, pre-sintering for 30 minutes to 96 hours at 900xc2x0 C. to 1300xc2x0 C. allows O2 gas in the sintering atmosphere to penetrate into pores of the molding and so is effective in promoting fineness of the sintered body.
Reasons for the restrictions in the composition
The reasons for the restrictions of X, Y in the compositional formula in the present invention to 0.3xe2x89xa6Xxe2x89xa61, 0.7xe2x89xa7Yxe2x89xa70 are that if X is less than 0.3 or Y exceeds 0.7, the Qf of the dielectric ceramic composition obtained is severely adversely affected, the frequency temperature coefficient departs considerably from 0 ppm/xc2x0 C., the improvement in sintering capability disappears, and it becomes impossible to control the temperature coefficient of frequency.
Also, by keeping Z and Zxe2x80x2 in a range 0 to 1, the resonance frequency temperature coefficient if can be selected at will in the range +4.0 to xe2x88x922.0 ppm/xc2x0 C.
Further, by keeping M in the range 0.2 to 0.8, the dielectric coefficient can be selected between 29 and 35, making it possible to match the size and coupling of various types of filter and/or electronic device. If M is less than 0.2, the temperature coefficient of frequency departs considerably from zero and if it exceeds 0.8 no improvement in the dielectric constant is seen.
The reason for restricting the concentration of O2 in the sintering atmosphere according to the present invention to 6% to 40% is that, under 6%, xcfx84f becomes low and the dielectric constant is excessively lowered, which is undesirable, and if it exceeds 40%, although this is desirable in terms of an improvement in the dielectric coefficient, xcfx84f becomes high and furthermore the Qf value falls, so this is also undesirable.
The reasons for restricting the pre-sintering temperature according to the invention to 900xc2x0 C. to 1300xc2x0 C. are that the molding becomes of largest size, due to thermal expansion, at 900xc2x0 C. to 1300xc2x0 C., enabling the oxygen and N2 to permeate to a fully sufficient extent into the interior of the molding. Outside this temperature range, permeation of O2 and N2 is incomplete, lowering the benefits thereof. If the pre-sintering time is less than 30 minutes, O2 and N2 cannot permeate into the middle of the molding and if it is more than 96 hours, evaporation of other constituents be comes severe, which is undesirable.
Also, the reason for restricting the sintering temperature to 1400xc2x0 C. to 1550xc2x0 C. is that if it is less than 1400xc2x0 C. sufficient fineness is not achieved and the dielectric constant becomes too high, while if it is more than 1550xc2x0 C., this causes deterioration of properties due to evaporation of Zn, which is undesirable. The reason for restricting the rate of rise of temperature to 50xc2x0 C./hour to 300xc2x0 C./hour is that, at less than 50xc2x0 C./hour, the total sintering time becomes too long, allowing constituents such as Zn to evaporate; if it is more than 300xc2x0 C./hour, cracks are produced in the sintered body, which is undesirable. The pre-sintering and sintering may be performed in the same pattern.
According to the invention, practically the same benefits may be obtained by substituting up to about 20 mol% of the Zn by divalent metal ions such as Ni2+, Co2+, or Mn2+ or alkaline earth ions such as Ca2+, or M 2+.